Microsoft has released Exchange On-Premises Mitigation Tool (EOMT) The tool is not a replacement for the Exchange security update but is the fastest and easiest way to mitigate the highest risks to internet-connected, on-premises Exchange Servers prior to patching. EOMT is a PowerShell script that must be run as Administrator on Exchange servers, and it does the following:Mitigates against current known attacks using CVE-2021-26855 via a URL Rewrite configuration. Uses Microsoft Safety Scanner to scan underlying Exchange Server for malicious artifacts and changes made by known threats.
Source: https://www.helpnetsecurity.com/2021/03/16/microsoft-exchange-mitigation/